It has long been known that increasing the compression ratio of an internal combustion engine increases its thermodynamic efficiency. The assumption used is that, for ideal cycles, a normally aspirated engine achieves full cylinder filling at atmospheric pressure. In this case the geometric volume ratio (of cylinder plus combustion chamber compared to combustion chamber alone) is designed to give a pre-combustion pressure just below the auto-ignition point of recommended fuels in gasoline engines, at the upper temperature regions of normal operation. For such engines operating at less than full load, however, the pressure of the mixture filling the cylinder is less than atmospheric. Consequently, while the geometric volume ratio remains constant, the pressure ratio is decreased, resulting in a mixture in the combustion chamber whose pressure is below that obtained at full throttle. One approach to restoring the full pre-combustion pressure (or achieving the desired pressure ratio) is to vary the volume of the combustion chamber. Designs to achieve this are numerous, but most lack the simplicity of design, and the robustness, to make their implementation practical. Some of the prior art is summarized below.
U.S. Pat. No. 741,824 by Pehrsson describes a 4-stroke, internal combustion engine with a cam operated exhaust valve and a vacuum operated intake valve, and with a main cylinder-diameter auxiliary piston used to manually vary the combustion chamber volume, to obtain the maximum pre-combustion pressure. The position of the auxiliary piston defines the limits of travel of the inlet valve.
In U.S. Pat. No. 1,167,023 by Schmidt, there is disclosed a conventional 4-stroke engine with an auxiliary cylinder and piston above the main cylinder, to vary the combustion chamber volume. The auxiliary piston position is controlled by a spring, and the load of combustion is taken up by hydraulic pressure. The position of the auxiliary piston is therefore dependent on the load, as the rate of supply and escape of oil from above the auxiliary piston (or the top half of it) is limited by the rate with which it can enter or escape through the oil ports. The outlet port has a valve that has a preset (and alterable) pressure relief system. If the combustion pressure exceeds this pressure, oil is forced out of the pressure chamber, and the auxiliary piston moves towards its outermost position, increasing the combustion chamber volume. When the load setting is decreased, and the combustion pressure is less than this value, oil is not released from the outlet, but still enters through the inlet check valve, resulting in the movement of the piston towards the main cylinder, and increasing the pre-combustion pressure.
Wilson discloses in U.S. Pat. No. 1,639,477 a conventional 4-stroke engine with an auxiliary cylinder and multiple crowned piston located above the working piston. The position of the auxiliary piston is controlled by the pressure in the intake manifold, via a second auxiliary piston, operated upon by hydraulic pressure. A spring is used to move the auxiliary piston to its outermost position when the engine is not running.
In U.S. Pat. No. 2,142,466 by Wagner a conventional 4-stroke compression ignition engine is described, with a throttled intake system and a variable volume combustion chamber. The auxiliary piston reciprocates, driven by the camshaft, whose timing is variable angularly.
In U.S. Pat. No. 2,344,993 by Lysholm there is disclosed a conventional 4-stroke spark or compression ignition engine with closure of the intake valve during the intake stroke, executing the Atkinson cycle. As the piston speed is high during valve closure, at high engine speeds the volumetric efficiency of the engine is reduced. If the intake valve were to be closed on the compression stroke instead, then the volumetric efficiency at high engine speeds would be increased, necessitating reducing it at low speeds. By closing the intake valve later on the induction stroke, and opening it very briefly on the compression stroke, the patent claims to produce a nearly constant volumetric efficiency, or that almost any desired curve of volumetric efficiency with engine speed can be tailored. On the compression ignition engine, the valve opening on the compression stroke can be the exhaust valve, as the fuel has not been injected as yet. The patent indicates that this is most suited to supercharged engines, thus in effect raising the efficiency to levels at or slightly above non-forced induction engines.
In U.S. Pat. No. 2,467,568 by Rosaen there is disclosed a conventional 4-stroke engine with a variable volume combustion chamber, controlled by hydraulic pressure. It appears that the auxiliary piston is filled with oil.
Humphreys discloses in U.S. Pat. No. 2,769,433 a conventional 2-stroke or 4-stroke engine with an auxiliary cylinder and piston located above the working piston. The auxiliary piston is backed by hydraulic fluid supplied by the engine's lubrication system. Oil can escape from the chamber at the back of the auxiliary piston if the maximum pressure achieved during combustion exceeds a preset value, which is enforced on the escape mechanism.
Heisling discloses in U.S. Pat. No. 2,883,974 a conventional 4-stroke engine with an auxiliary cylinder and piston located above the working piston, whose purpose is to provide a variable volume combustion chamber. The position of the auxiliary piston is controlled by hydraulic pressure, and it is also actuated by hydraulic pressure. A second embodiment utilizes a moving cylinder sleeve. Much of the emphasis of the invention is on the control mechanism to make the idea work.
In U.S. Pat. No. 3,964,452 by Nakamura an auxiliary cylinder and piston are located above a main cylinder and piston, and may communicate with them. A spring loaded piston may slide in the auxiliary cylinder, and does so after a certain preset high pressure is achieved during combustion. The primary purpose is to limit pressures and temperatures during combustion, and therefore increase thermal efficiency while ensuring that production of NO.sub.x is limited. Lean charges lead to reduced CO and HC emissions, but they may also produce misfires and combustion fluctuations.
In U.S. Pat. No. 4,033,304 by Luria there is described an internal combustion engine that achieves constant pre-combustion pressure with a movable auxiliary piston and cylinder, wherein said piston is controlled by hydraulic means. The invention implements the Atkinson cycle with variable inlet valve timing. The inlet valve is held open past bottom dead center ("BDC") to vent the unwanted mixture back into the inlet tract. The auxiliary piston is spring loaded so that during the exhaust stroke, the auxiliary cylinder volume is at its least, thereby aiding scavenging.
In U.S. Pat. No. 4,187,808 by Audoux there is disclosed a conventional 4-stroke engine, with an auxiliary piston and cylinder located above the main piston. The focus is on the hydraulic valving used to reduce any loads on the mechanical actuation of the auxiliary piston. With lowered pressure in the main cylinder, the auxiliary piston is designed to move inward towards the main cylinder, decreasing the volume of the combustion chamber, and assisting in scavenging burnt gases.
U.S. Pat. No. 4,516,537 by Nakahara discloses a conventional 4-stroke engine with an auxiliary cylinder and piston located in the head above the main piston and cylinder. The auxiliary piston is moved by hydraulic fluid under pressure. A major thrust of the invention is to construct a feasible means of controlling the auxiliary piston position, without having the excessive pressures from combustion having an impact on the oil pressure system, yet to distribute the load uniformly.
U.S. Pat. No. 4,798,184 by Palko discloses a 2-stroke or 4-stroke diesel engine where the intake valve is closed on the compression stroke to implement a greater expansion duration than compression. The valve actuation means is not described in any detail, and the patent indicates that it may be fixed or it may be variable during operation. The extended expansion allows combustion to occur at or after top dead center ("TDC"), reducing engine loads, and heat losses to the cooling system. The usual advantages to the expansion increase are more work produced by the expanding gases, and lower exhaust temperatures.
The prior art approaches for varying the volume of the combustion chambers can be placed into four main categories, each of which has advantages and disadvantages:
(1) Move the centerline of the crankshaft relative to the line running parallel with the center of the head. This has the advantage that the conventional combustion chamber shape and volume can be used. The disadvantage is that the system must be complex to control accurate relative movement of the crankshaft and its crankcase, and the remainder of the engine. Sealing such a mechanism against oil leakage is difficult. The design may cause problems with V-shaped engine blocks, and will not work with rotary or orbital engine designs. PA0 (2) Split the connecting rod so that its length can be varied. There are two main variants of this idea: one is to hydraulically control the length, which is adjusted only with varying throttle settings, and the other is to provide a spring between the two portions of the connecting rod. Both increase the reciprocating mass of the engine, and make balancing of components more difficult. The sprung connecting rod exacerbates the balance problems, leading to considerable vibration in the engine. The advantage of such a design is the same as for (1). This will not work with rotary or orbital engines. PA0 (3) Split the piston, much like the connecting rod, so that the distance from the center of the crankshaft to the piston crown may vary. This has the same problems as the split connecting rod, and the same advantage. PA0 (4) Provide a means of varying the shape of the combustion chamber, and leave the piston, connecting rod and crankshaft the same as in a conventional engine. The means of varying the volume include an auxiliary cylinder and piston, or a flexible diaphragm. The disadvantage of the diaphragm is that work hardening may occur of the flexible material, leading to failure that will prevent the engine from working correctly. The auxiliary cylinder and piston has several forms. These include straight mechanical actuation (regardless of the means of sensing the correct position), hydraulic actuation that also acts to absorb combustion pressures, and mechanical/electrical actuation, with hydraulic shock absorption. Of the three types cited, the last one provides the most accurate means of positioning, and achieves (along with the second means) suitable protection of the mechanism from the forces experienced during combustion. PA0 (1) Combustion forces on the face of the auxiliary piston do not cause damage or significant wear to the actuation mechanism. PA0 (2) The auxiliary piston can be withdrawn, to give the maximum combustion chamber volume, in the event of the ignition being switched off, the engine not running, or failure of the control mechanism. PA0 (3) Withdrawal of the auxiliary piston can be achievable in the same period of time that the operator of such an engine can alter the load setting from idle to full load. PA0 (4) Input information for the determination of the position of the auxiliary piston can be readily available from the sensors used to control fuel injection. PA0 (5) In the case of hydraulic actuation, the fluid supply has a check valve to prevent the high combustion chamber pressures from affecting the remainder of the fluid system (be it engine oil or a separate supply). PA0 (6) There is a cushion of oil trapped between the auxiliary piston and the cylinder when the piston is retracted to its uppermost position. This will prevent collision of the piston and cylinder. PA0 (7) The back side of the auxiliary piston is cooled. This is achieved by continuous circulation of lubricating oil in the chamber between the piston and its cylinder. PA0 (8) Under constant load, oil can discharge through a long orifice, in such a way that the combustion pressures subside before there is immoveable contact between the auxiliary piston and the positioning rod. PA0 (9) Such a long orifice can be located at the highest point within the auxiliary cylinder, so that any bypass gases may be vented. PA0 (10) There is a stop at the end of the auxiliary cylinder facing the working cylinder, to prevent the auxiliary piston from entering the working cylinder, in the event of failure.